1. Field of the Invention
This invention relates to a recording apparatus and, more particularly, to a type of recording apparatus that has an impact recording head which drives recording elements by using electromagnets.
2. Description of Prior Art
A known type of recording apparatus, such as a character printer or wire dot printer, performs recording in such a manner that recording elements are driven by solenoids so as to impact against a recording medium. In this type of apparatus, a solenoid coil is supplied with a driving current, and a recording element such as a wire or hammer is made to abruptly move forward and contact the recording medium. Switching elements such as transistors are used to drive the solenoid.
If it is necessary to accurately control and set the recording pressure to a desired intensity or to an intensity that accords with the types of character and recording medium, the driven state of the solenoid must be detected.
However, in a typical conventional apparatus, the solenoid coil cannot be controlled in the above described manner since coil drive conditions including the drive timing and the drive voltage and current are previously set within fixed ranges. In a rare type of apparatus, the drive current flowing through the coil is detected and the coil drive conditions are controlled in accordance with the value thereby detected. On the other hand, almost all impact recording heads of the electromagnetic drive type are controlled by open loops.
A method which involves measuring the magnetic flux of the coil, which is closely related to the attracting force generated by the solenoid, is one of the methods that are capable of controlling the recording pressure of the impact recording head most accurately. However, it is not possible for the above-mentioned types of conventional apparatus to perform accurate driving force control since they are based on an open-loop control method or a method of detecting the coil current.
In addition, in the case of solenoids used with the conventional impact heads, etc., residual magnetic flux is observed after driving due to eddy currents which occur in the core of the coil or to the hysteresis characteristics of the electromagnetic circuit. Therefore, there is a problem of delay in the advancing or returning movement of the armature due to such residual magnetic flux, or a problem of errors in the driving forces due to magnetic interference between the solenoids.